This invention relates to fuel-circulating fuel cell systems and more particularly to a hydrogen-circulating mechanism in fuel cell systems.
In the fuel cell system, hydrogen and air should be supplied more than fuel cells consume, so as to discharge condensed water in the fuel cells. Hydrogen is supplied from storage equipment such as a cylinder installed in a vehicle, and thus discharging unused hydrogen into the air would conspicuously impair the fuel efficiency of hydrogen. For that reason, systems for circulating unused hydrogen utilizing a pump or the like have been devised.
Methods for recycling hydrogen unused in a fuel cell fall roughly into two types.
The first type is a method of circulating hydrogen utilizing a fuel pump, in which an operating section that rotates and/or slides is used to collect and feed fuels. This type will be hereinafter referred to as “fuel pump-using approach”. However, this method would disadvantageously require a fuel pump of large size and high power consumption for driving the fuel pump, thus decreasing the fuel efficiency of a fuel-cell vehicle. Another problem associated with the fuel pump-using approach is that the pressure of hydrogen in a high-pressure hydrogen tank could not be utilized effectively.
The second type is a method of circulating hydrogen utilizing an ejector, or a kind of jet pump. This type will be hereinafter referred to as “ejector-using approach”. This method employs energy generated from a high pressure in the high-pressure hydrogen tank to circulate hydrogen, and thus no power consumption is required. With the ejector-using approach, however, since circulation of hydrogen will not take place until the fuel cells consume hydrogen, the circulation rate of hydrogen would disadvantageously decrease when the output of the fuel cells is reduced. Further, the ejector has a nozzle inserted to generate velocity of hydrogen, which would thus cause a delay of the ejector in responding to an abruptly increased output of the fuel cells, with the result that the circulation rate disadvantageously could not reach a target value.
The problem encountered in the ejector-using approach will now be described in detail with reference to FIG. 20. FIG. 20 is a block diagram of a prevailing hydrogen-circulating system that only uses an ejector as a means for circulating hydrogen.
In this system, hydrogen supplied from a high-pressure hydrogen tank 101 undergoes pressure regulation by a regulator 102, and is then ejected by an ejector 103 to a fuel cell stack 104. The fuel cell stack 104 has been supplied with excessive amounts of hydrogen to discharge condensed water as described above. Hydrogen unused in the fuel cell stack 104 flows through a hydrogen-circulating passage 106 into the ejector 103, and circulates through the system together with hydrogen supplied from the high-pressure hydrogen tank 101. The system is configured to increase hydrogen pressure exercised upon the fuel cell stack 104 as the output of the fuel cell stack 104 increases.
In this system, when an abrupt acceleration instruction is given to the fuel cell stack 104, a huge amount of hydrogen is consumed rapidly in the fuel cell stack 104, and thereby the hydrogen pressure in the fuel cell stack 104 decreases. If hydrogen enough to make up the consumed amount could immediately be supplied, no problem would arise. However, there is a time lag between the decrease of pressure in the fuel cell stack 104 and the decrease of pressure transmitted through a hydrogen flow passage 107 to the ejector 103, and the response of the ejector 103 to the decrease of pressure in the fuel cell stack 104 would eventually delay. Moreover, the hydrogen is supplied through a narrowed nozzle of the ejector 103, and thus a predetermined period of time is required until the amount of hydrogen needed to be supplied to the fuel cells is reached.
The above situation is shown in FIG. 21A, which depicts an amount of hydrogen needed to be supplied to the fuel cell stack 104 (indicated by a broken line), and an amount of hydrogen actually supplied through the ejector 103 (indicated by a solid line), when an abrupt acceleration instruction is transmitted to the fuel cell stack 104.
As shown in FIG. 21A, after an abrupt acceleration instruction is provided, the amount of hydrogen to be supplied to the fuel cell stack 104 increases rapidly, but the amount of hydrogen actually supplied through the ejector 103 does not follow the rapid increase, resulting in deficiency of hydrogen (or so-called “hesitation”). The hesitation causes damage such as a rupture of electrolyte membranes of the fuel cells, and would lead to a destruction of the fuel cells if the worse came to the worst.
Further, when an abrupt deceleration instruction is given to the fuel cell stack 104, the system should control a hydrogen pressure exercised upon the fuel cell stack 104 to be reduced to a predetermined level. To that end, supply of hydrogen from the high-pressure hydrogen tank 101 to the fuel cell stack 104 is stopped, so that hydrogen in the fuel cell stack 104 is consumed. However, the stop of supply of hydrogen, which should have been introduced into the ejector 103, would disadvantageously disable the ejector 103 from circulating hydrogen. If hydrogen were not circulated smoothly, condensed water would accumulate in the fuel cell stack 104, which would thus decrease a cell voltage, and could lead to damage the fuel cell stack 104 at worst.
The above situation is shown in FIG. 21B, which depicts an amount of hydrogen needed to be supplied to the fuel cell stack 104 (indicated by a broken line), and an amount of hydrogen actually supplied through the ejector 103 (indicated by a solid line), under conditions where an abrupt deceleration instruction is given to the fuel cell stack 104.
When an abrupt deceleration instruction is given to the fuel cell stack 104, the fuel cell stack 104 needs hydrogen circulation as shown in the broken line, but in actuality the ejector 103 stops and no longer capable of circulating hydrogen, and thus hydrogen is held in the system. Under these conditions, condensed water generated in the fuel cell stack 104 cannot be removed; consequently, the condensed water accumulates in the fuel cell stack 104.
Moreover, as the conditions of the fuel cell stack 104 continuously change, operation of the hydrogen circulation system is disadvantageously unable to be optimized for the system unless the hydrogen circulation system is controlled in accordance with the varying conditions.
The present invention is made to eliminate the above-discussed disadvantages.